1. Field of the Invention
The present invention relates to a solar cell and a method of manufacturing the same, and more particularly, to improvement of a solar cell used in a solar cell module including a plurality of series-connected solar cells and a method of manufacturing the same.
2. Description of the Background Art
FIGS. 19 and 20 show a light receiving surface (also referred to as "front plane") and a back surface of a conventional solar cell. A solar cell 10 includes a semiconductor substrate 1. The front plane of substrate 1 is covered with an anti-reflection film 3. A comb-like front silver electrode is formed on anti-reflection film 3. The front silver electrode is covered with a solder layer 8a. A major portion of the back surface of solar cell 10 is covered with an aluminum electrode 6. Back silver electrodes are formed at a plurality of local regions in the region of the aluminum electrode 6. The back silver electrodes are covered with a solder layer 5a.
FIGS. 21 and 22 illustrate a method of manufacturing the solar cell shown in FIGS. 19 and 20. For the sake of simplification, the dimensions between various elements shown in the drawings are not drawn to scale.
It is to be noted that FIGS. 21(A) to (D) and FIG. 22(A) to (D) show sequential steps of one manufacturing process.
Referring first to FIG. 21(A), a semiconductor substrate 1 is prepared having, for example, a diameter of 100 mm, and a thickness of 0.4 mm. A P-type semiconductor substrate 1 generally has an impurity concentration in the range of 3.times.10.sup.15 to 4.times.10.sup.16 cm.sup.-3. A silicon substrate 1 having a (100) main surface is generally used. Preferably, the light receiving surface of the silicon substrate 1 is formed in a pyramid-like concave and convex manner (referred to as a "textured surface") as in the circle showing an enlarged portion thereof, for the purpose of reducing light reflection. Such a textured surface is formed by treating the silicon substrate 1 for 20 to 30 minutes at a temperature in the range of 80.degree. C. to 90.degree. C. while adding isopropyl alcohol in a solution including several % of NaOH.
Referring next to FIG. 21(B), an n.sup.+ layer 2 having a thickness of approximately 0.4 .mu.m, for example, is formed all over the surface of the silicon substrate 1. An n.sup.+ layer 2 can be formed by applying a diffusion process for 45 minutes at 900.degree. C. in an ambient including POCl.sub.3 gas, for example. Here, a phosphorus glass film (not shown) is produced on the surface of n.sup.+ layer 2 which is not required. This phosphorus glass film can be removed by dipping the same in a solution including 10% of HF for one minute.
Referring to FIG. 21(C), an anti-reflection film 3 such as titanium oxide or silicon oxide is formed by evaporation or CVD on the light receiving surface of the substrate 1. An anti-reflection film 3 is formed to a thickness of 70 to 80 nm. The presence of an n.sup.+ layer 2 all over the surface of silicon substrate 1 causes shorting between the negative voltage (n side) and the positive electrode (p side) of the solar cell, so that favorable electrical characteristics cannot be obtained. It is therefore necessary to the n.sup.+ layer 2 at least from the back surface of silicon substrate 1.
Referring to FIG. 21(D), an acid resistant resist 4 is applied on the anti-reflection film 3 by screen printing and then dried. Then, an etching step is applied using a mixture of hydrofluoric acid and nitric acid (HF: HNO.sub.3 =1:3), whereby the n.sup.+ layer 2 is removed from the side face and back surface of the substrate 1.
Then, the resist layer 4 is removed using a solvent such as toluene or xylene as shown in FIG. 22(A). Referring to FIG. 22(B), paste 5 including silver and paste 6 including aluminum are printed at a predetermined pattern and then dried. The substrate 1 is subjected to a thermal treatment at 700.degree. C. to 800.degree. C. whereby a back silver electrode 5 having a thickness of approximately 20 .mu.m and a back aluminum electrode 6 having a thickness of approximately 50 .mu.m are formed by baking. Here, aluminum and silicon are alloyed, whereby a p.sup.+ layer 7 is formed under the aluminum electrode 6. The p.sup.+ layer 7 has a thickness of approximately 5 .mu.m to induce a BSF (Back Surface Field) effect.
Referring to FIG. 22(C), a paste 8 including silver is printed at a predetermined pattern on the anti-reflection film 3 and then dried. Then, a thermal treatment is applied to the substrate 1 at a temperature in the range of 600.degree. C.-700.degree. C., whereby a front silver electrode 8 having a thickness of approximately 20 .mu.m is formed by baking. Here, the silver paste includes glass frit, and the front silver electrode 8 forms an ohmic contact with the n.sup.+ layer 2 through the anti-reflection layer 3.
Referring to FIG. 22(D), solder layers 5a and 8a having a thickness of approximately 20 .mu.m are formed on the surfaces of the back silver electrode 5 and front silver electrode 8, respectively. Thus, a solar cell 10 is completed. The structure shown in FIG. 22(D) corresponds to the structure of FIG. 20 taken along line 24H--24H.
Although the above description is made of an n.sup.+ layer 2 formed by diffusion using POCl.sub.3 gas, an n.sup.+ layer can be formed on a light receiving surface by applying a dopant solution including alkyl silicate, alcohol, carboxylic acid, etc. and phosphorus pentoxide as a diffusion source on the light receiving surface of the silicon substrate by a spin coater, which is then subjected to a diffusion thermal treatment. However, this known method also results in formation of an n layer at the back surface and side face of the silicon substrate due to auto doping caused by out-diffusion from the applied doping agent. Therefore, this n layer must be removed as in the step shown in FIG. 21(D) by a resist printing method.
The above-described conventional method of manufacturing a solar cell requires various steps such as a resist printing method to remove unrequired regions of the n type layer, an etching step, and a resist removal step. Therefore, the conventional manufacturing process of a solar cell is expensive.
Furthermore, a conventional solar cell manufactured as described above has problems as set forth in the following.
A solar cell is seldom used singly, and a solar cell module is generally used in which a plurality of solar cells are connected in series, as shown in FIG. 23(A).
FIG. 23(A) shows the top plan view of a solar cell module including thirty six solar cells 10 connected in series. A solar cell 10 is connected to an adjacent solar cell 10 in series by an interconnector 11.
FIG. 23(B) shows a cross section structure of the solar cell module of FIG. 23(A) taken along line 23B--23B. The solar cell module includes a support plate 12 of transparent tempered glass. The solar cells 10 connected in series by interconnectors 11 are imbedded within an EVA resin layer 13. A white weather resistant film 14 covers the bottom surface of the EVA resin layer 13.
FIG. 23(C) is an equivalent circuit diagram of a solar cell module having a plurality of solar cells connected in series. In FIG. 23(C), the small arrows represent light entering a solar cell, and the long arrow I represents the direction of the output current of the solar cell module.
When a solar cell module is actually used, a portion thereof may be shadowed. More specifically, the shadow of a tree, a building, or electric wires is cast on the solar cell module. There is also a possibility of the solar cell module being shadowed by droppings of birds or dust adhered to the surface thereof.
In a mode where a short-circuit is established across solar cells at both ends of the solar cell module, voltage generated from a solar cell which is not shadowed is applied to a solar cell which is shadowed as a reverse bias voltage. In such a shadowed solar cell, power caused by current according to the reverse bias voltage generates heat to be consumed. When this reverse bias voltage exceeds the peak inverse voltage of the solar cell, short-circuit breakdown occurs in that solar cell, whereby the output characteristics of the entire solar cell module are significantly degraded. The rise in temperature and the short-circuit breakdown of a shadowed solar cell depends upon the reverse direction properties of a solar cell. It is preferable to facilitate current flow in the reverse direction of a solar cell in order to reduce such a phenomenon.
In a conventional solar cell as shown in FIG. 22(D), complete isolation is achieved by a pn junction between the positive electrode side and the negative electrode side. Therefore, the forward direction properties of the solar cell are favorable so that a high conversion efficiency can be obtained. However, current does not easily flow in the reverse direction.
FIG. 24 shows the current voltage (I-V) characteristics of such a solar cell in a qualitative manner. The voltage V is plotted along the abscissa, and the current I is plotted along the ordinate. Curve 24A shows the I-V characteristics of a solar cell illuminated with light, and curve 24B shows the same in a solar cell in a darkened state. It is appreciated that reverse direction current cannot easily flow in a solar cell as shown in FIG. 22(A) in a darkened state.
FIG. 25 shows an equivalent circuit diagram of a solar cell. It is considered that a solar cell includes parallel resistance and serial resistance. More specifically, it is expected that a solar cell having the I-V characteristics as shown in FIG. 24 has great parallel resistance. It is to be noted that the parallel resistance and serial resistance are not shown in the equivalent circuit diagram of FIG. 23(C).
FIG. 26 is a graph showing the influence of a shadow in a solar cell module including thirty six solar cells, each having a diameter of 100 mm, and a parallel resistance of 20 k.OMEGA./cm.sup.2. The output voltage of the solar cell module is plotted along the abscissa, and the output current is plotted along the ordinate. The curve representing 100% shows the I-V characteristics of a solar cell module in which the light receiving surface of one solar cell is entirely shadowed out of the thirty six solar cells. Similarly, the various % numerics corresponding to respective curves represent the amount of the shadowed area formed on one solar cell out of the thirty six solar cells. It is appreciated from FIG. 26 that the entire output of a solar cell module is extremely reduced as the area of a shadow formed on one solar cell increases when each solar cell has great parallel resistance.
FIGS. 27(A) and (B) show the result of a simulation in assessing the power consumption of one shadowed solar cell in a solar cell module where thirty two solar cells having a parallel resistance of 20 k.OMEGA./cm.sup.2 are connected in series. In FIG. 27, the curve 27B shows the I-V characteristics of one solar cell in which 20% of the light receiving area is shadowed. Curve 27A shows the I-V characteristics of the remaining thirty one solar cells connected in series. Curve 27C shows the I-V characteristics obtained by combining curves 27A and 27B. The area of the hatched region corresponds to power consumed by the one shadowed solar cell.
Referring to FIG. 27(B), curve 27D shows the I-V characteristics of one solar cell in which 70% of the light receiving area is shadowed. Curve 27(E) shows the I-V characteristics obtained by combining curves 27A and 27D. By comparing the area of the hatched region between FIG. 27(A) and FIG. 27(B), it is appreciated that the power expended by a shadowed solar cell having a shadowed light receiving area of 70% is lower than that having a shadowed light receiving area of 20%.
FIGS. 28(A) and (B) are similar to FIGS. 27(A) and (B), but wherein each solar cell has a parallel resistance of 1 k.OMEGA./cm.sup.2. Referring to FIG. 28(A), curve 28B shows the I-V characteristics of one solar cell in which 20% of the light receiving area is shadowed. Curve 28A shows the I-V characteristics of the remaining thirty one solar cells. Curve 28C shows the I-V characteristics obtained by combining curves 28A and 28B. Referring to FIG. 28(B), curve 28D shows the I-V characteristics of one solar cell in which 70% of the light receiving area is shadowed. Curve 28E shows the I-V characteristics obtained by combining curves 28A and 28D. It is appreciated by comparing the area of the hatched regions between FIG. 28(A) and FIG. 28(B) that power expended by one shadowed solar cell having a shadowed light receiving area of 70% is greater than that having a light receiving area of 20%, contrary to FIGS. 27(A) and (B).
FIG. 29 is a graph showing a result of a wider range of such a simulation shown in FIGS. 27(A), (B) and FIGS. 28(A), (B). In the present graph, the ratio of a shadow formed on the light receiving surface of one solar cell is plotted along the abscissa, and a power (W) expended by one shadowed solar cell is plotted along the ordinate. Curves 29A, 29B, and 29C correspond to a solar cell having parallel resistance of 20 k.OMEGA./cm.sup.2, 1 k.OMEGA./cm.sup.2, and 100 k.OMEGA./cm.sup.2, respectively. It is appreciated that the increase in temperature of a shadowed solar cell is greater as the power consumption is increased.
FIG. 30 is a graph showing the increase in temperature of one shadowed solar cell in a module having thirty six solar cells connected in series. The ratio of shadowed area to the light receiving surface is plotted along the abscissa, and the increase in temperature (.degree. C.) of a shadowed solar cell is plotted along the ordinate. Curve 30A shows the increase in temperature when the solar cell has a parallel resistance of 20 k.OMEGA./cm.sup.2. It is appreciated from curve 30A that the temperature of a shadowed solar cell is higher by 72.degree. C. than other solar cells when 20% of the light receiving area is shadowed in the shadowed solar cell. (Curve 303 will be explained later).
FIG. 30 relates to a solar cell module including thirty six solar cells connected in series. The increase in temperature of a shadowed solar cell will be further increased in a solar cell module including a greater number of solar cells. In practice, there is sufficient possibility of 20% of the light receiving area of one solar cell being shadowed.
Thus, it is understood that local shadowing in a solar cell module causes significant reduction in the output of the entire module. There is possibility of a shadowed solar cell being heated excessively so as to be damaged. In the worst case, a fire may break out. For example, in fine weather, the entire solar cell module rises to the temperature of 60.degree. C. to 70.degree. C. by solar heat. According to the example shown in FIG. 30, a solar cell having 20% of the light receiving area shadowed may be heated up to 132.degree. C. to 142.degree. C. In such a case, there is possibility of the EVA resin in which solar cells are imbedded being colored or pores being generated therein.
In order to prevent damage due to heating of such a shadowed solar cell, a solar cell module as shown in FIGS. 31(A), (B) and (C) has been proposed. The solar cell module of FIG. 31(A) has rectangular solar cells 10 connected in series by interconnectors 11. FIG. 31(B) is an enlarged perspective view of the portion indicated by the circle in FIG. 31(A). More specifically, adjacent solar cells 10 are connected by an interconnector 11 via a bypass diode 15. FIG. 31(C) is an equivalent circuit diagram of a solar cell 10 including the bypass diode 15 of FIG. 31(B) (parallel resistance and serial resistance are not shown). It is appreciated from the circuit diagram that the bypass diode 15 passes current caused by reverse bias voltage applied to a shadowed solar cell 10. Therefore, excessive heating or a short circuit breakdown can be prevented in a shadowed solar cell. However, the solar cell module of FIGS. 31(A), (B), and has the disadvantage that the process of connecting the plurality of solar cells while attaching a bias diode is very complicated. Therefore, the manufacturing cost thereof is expensive.
The usage of solar cells having integrated bypass diodes (Japanese Patent Laying-Open No. 3-24768) and solar cells integrated with zener diodes connected in parallel with the same polarity (Japanese Patent Laying-Open No. 5-110121) are known for solar cell modules. However, such solar cells must have diodes formed therein using mask alignment, which is a complicated manufacturing process of the solar cell. This results in an increase of the manufacturing cost.